1. Field
The present invention relates to a fuel cell, particularly, to a fuel cell including a fuel electrode having an improved diffusion layer.
2. Description of the Related Art
A direct-methanol-type fuel cell (DMCF) is constructed to comprise a unit cell including a membrane-like electrode unit, a fuel passageway plate arranged on one surface of the membrane-like electrode unit, and an oxidizing gas passageway plate arranged on the other surface of the membrane-like electrode unit. The membrane-like electrode unit comprises a fuel electrode to which is supplied a methanol aqueous solution as a fuel, an air electrode to which is supplied an oxidizing gas, and a polymer electrolyte membrane interposed between the fuel electrode and the air electrode. The fuel electrode includes a catalyst layer and a diffusion layer having a carbon paper prepared by laminating carbon fibers and arranged on the catalyst layer, the catalyst layer and the diffusion layer being arranged in the order mentioned on one surface of the electrolyte membrane. The air electrode includes a catalyst layer and a diffusion layer having a carbon paper prepared by laminating carbon fibers and arranged on the catalyst layer, the catalyst layer and the diffusion layer being arranged in the order mentioned on the other surface of the electrolyte membrane.
When electric power is generated in the fuel cell of the construction described above by supplying an aqueous solution of methanol used as a fuel into the fuel electrode and by supplying an oxidizing gas such as air into the air electrode, the diffusion layer of mainly the fuel electrode, the diffusion layer having a carbon paper, tends to become clogged with water by capillary action. The diffusion layer clogged with water loses its function as a fluid passageway for allowing the methanol aqueous solution used as a fuel to flow toward the catalyst layer. As a result, the catalyst layer partly fails to have the methanol aqueous solution, which is used as a fuel, supplied thereto, lowering the output of the fuel cell.
Such being the situation, Japanese Patent Disclosure (Kokai) No. 2004-234947 discloses a diffusion layer having a carbon paper prepared by laminating carbon fibers each having the surface covered with a fluoroplastic material such as polytetrafluoroethylene.
However, a carbon fiber processed with a fluoroplastic material exhibits an excessively strong water repellency so as to make it difficult to allow the aqueous fuel to flow into the catalyst layer through the carbon paper. Also, the aqueous fuel once taken into the catalyst layer is locked in by the strong water repellency of the carbon fiber. As a result, it is difficult for the aqueous fuel to flow from the catalyst layer to the outside through the diffusion layer. It follows that it is difficult to allow the aqueous fuel to flow into the catalyst layer in a sufficiently large amount, with the result that the output of the fuel cell is lowered.